1. Field of the Invention
The invention relates to apparatus for producing an electromagnetic beam in space having spatial encoding thereof and more particularly to missile guidance systems of the beamrider type.
2. Description of the Prior Art
A beamrider guidance system functions to maintain missile line of flight in a desired direction. Such systems are most readily applied to short range missile guidance problems and have found particular applications in surface to surface (primarily anti-tank) and surface to air (primarily short range air defense) missions.
A beamrider system generally includes a transmitting section and receiving section, with the receiving section being located on board the missile. In operation, an observer locates a target and projects a beam of electromagnetic radiation from the transmitter to the target. The beam of electromagnetic radiation may be viewed as a volume of radiation forming a guidance corridor to the target which, if followed by the missile, will cause it to strike at the desired location. To assure missile impact on the target, it is necessary for the missile, launched into the beam, to have means for sensing its position within the radiated beam and for controlling its velocity vector to be closely aligned with the beam axis during the flight.
This task may be accomplished by spatially modulating the beam at the transmitter, which modulation is detected and decoded at the missile receiver. The decoded modulation may then provide on board electronics with data indicative of missile position relative to the beam axis. The position data may be used to generate error signals for use by missile guidance devices to steer the missile along the beam axis. More specifically, spatial modulation of the guidance beam results in the formation of an illumination pattern over a cross section of the beam. The illumination pattern divides the beam into a series of resolution elements, with each resolution element bearing a unique signature by reason of its modulation. The missile locates itself relative to the beam axis by detecting the modulation from the resolution element in line with its receiver.
It is known to spatially modulate the electromagnetic radiation beam of a beamrider guidance system in amplitude or frequency. Basic encoding mechanisms include analog AM, digital AM and analog FM modulation. In my co-pending U.S. patent application Ser. No. 7,751, now U.S. Pat. No. 4,299,360, I disclose an encoding technique which is referred to as digital FM encoding. Examples of other known beam modulation techniques can be found in Menke U.S. Pat. No. 3,690,594, issued Sep. 12, 1972; Miller, Jr., et al U.S. Pat. No. 3,782,667, issued Jan. 1, 1974; MacLusky U.S. Pat. No. 3,501,113, issued Mar. 17, 1970; and Hawes U.S. Pat. No. 3,255,984, Hawes, issued Jun. 14, 1966. In addition, Esker, et al U.S. Pat. No. 4,014,482, teaches pulsed laser spatially modulated beam and Glenn U.S. Pat. No. 4,174,818 shows a digital AM dependent on the amplitude of transmitted pulses.
Amplitude modulation techniques for beamrider guidance systems are exemplified by the aforementioned patent to Hawes and in an embodiment of the beamrider guidance system disclosed in the aforementioned Miller, Jr., et al patent. A beamrider guidance system which uses amplitude modulation techniques, be it analog AM or digital AM, suffers from amplitude fluctuations caused by both natural atmospheric scintillations and pertubations caused by missile wake and plume.
Known beamrider guidance systems using analog frequency modulation techniques have overcome problems associated with amplitude modulated guidance systems. However, these frequency modulation systems are susceptible to noise problems, making frequency discrimination oftentimes difficult. An additional problem with frequency modulation type beam guidance systems are that they are complex. They often require multiple radiation sources to provide a beam having a frequency coded illumination pattern over its cross section, as well as mechanically complicated rotating conical scanners to cause nutation of the transmitted beam which allows a single detector at the missile carried receiver to properly locate the missile relative to the beam axis. A better understanding of these complex frequency modulation guidance systems will be had upon review of the systems described in the aforementioned Miller, Jr., et al and Menke patents. The Esker, et al patent describes a missile directing system utilizing a continuously variable frequency code.
In a conventional frequency modulation technique for spatially encoding a cross section of a guidance beam of a beamrider system, such as that illustrated in the aforementioned Miller, Jr., et al patent, the guidance beam is frequency divided into four quadrants by using four radiation sources, each of a different frequency. The modulated radiation from the four sources are confined into a single beam having the desired spatial modulation by directing the radiation from the four radiation sources through light pipes to a light pipe common junction. The combined radiation is transmitted to nutation projection optics for transmitting the beam to the target.
The target, which may be a missile, is provided with a single detector and cooperating receiving circuitry designed to calculate the time during which each modulation frequency is received at the missile detector during a beam nutation cycle. The missile is properly aligned to the beam axis when the detector receives each frequency for the same period of time during a single nutation cycle. The above described system may be termed an analog frequency modulation beam guidance system.
Another technique for providing analog frequency modulation to a guidance beam of a beamrider guidance system is illustrated in the aforementioned Menke patent. Menke develops frequency modulation of a guidance beam by nutating a rotating disc divided into a number of radiation transmitting pie-shaped sections and a like number of alternately arranged radiation opaque pie-shaped sectors. The sectors are shaped in the described manner so that the width of each sector at a point close to the disc center is less than the sector width at the disc perimeter. The disc is rotated in the path of a guidance beam thereby imparting frequency modulation to the beam. More specifically, the rotating disc functions to chop the guidance beam such that the rotating disc projects an image pattern across the beam cross section, which pattern may be visualized as a series of different frequency divisions extending across the beam cross section. When the rotating disc is nutated, a single detector only is required for locating the missile relative to the beam axis.
The present invention is directed to an improved beamrider guidance system using phase modulation techniques for spatially encoding the guidance beam. As will become evident from a reading of a description of the invention set out hereinafter, the improved guidance system which uses digital phase modulation encoding eliminates the complexity of known FM type beam guidance systems and represents an improvement over the digital frequency modulation type guidance system of my above referenced co-pending patent application.
The novel phase modulation encoding may be implemented with fewer components than my frequency modulation encoding system and has been found to have about 3 dB improvement in signal to noise ratio.